The invention relates to a process for the preparation of hydrocarbons having at least five carbon atoms per molecule.
Hydrocarbons of at least five carbon atoms per molecule (hereinafter referred to as "C.sub.5.sup.+ hydrocarbons") can be prepared from hydrocarbons having at most four carbon atoms per molecule (hereinafter referred to as "C.sub.4.sup.- hydrocarbons") by a two-step process in which in the first step the C.sub.4.sup.- hydrocarbons are converted by steam reforming into a mixture of carbon monoxide and hydrogen, which mixture is contacted in the second step at elevated temperature and pressure with a catalyst and thus converted into a mixture of hydrocarbons consisting substantially of C.sub.5.sup.+ hydrocarbons. The reaction which takes place in the second step of the process is known in the literature as the Fischer-Tropsch hydrocarbon synthesis. Catalysts often used for this reaction contain one or more metals from the iron group together with one or more promoters and a carrier material.
A catalyst's usefulness for the preparation of C.sub.5.sup.+ hydrocarbons from H.sub.2 /CO mixtures is mainly determined by the catalyst's activity, C.sub.5.sup.+ selectivity and stability, the catalyst being regarded as more useful according as these parameters have a higher value. In the preparation of C.sub.5.sup.+ hydrocarbons from C.sub.4.sup.- hydrocarbons according to the above-mentioned two-step process the catalyst's stability draws most emphasis. For according as the catalyst has a higher stability, the process can be carried out for a longer period before it becomes necessary to replace the catalyst. It is true that, according as the catalyst has a lower activity, less of the H.sub.2 /CO mixture will be converted per reactor throughput, and more C.sub.4.sup.- hydrocarbons will be formed as by-product according as the catalyst has a lower C.sub.5.sup.+ selectivity, but by recycling unconverted H.sub.2 and CO and also by recycling the C.sub.4.sup.- hydrocarbons formed as by-product a high conversion of the H.sub.2 /CO mixture and a high C.sub.5.sup.+ selectivity can be realized all the same. Thanks to the possibilities of compensating for lower activity and C.sub.5.sup.+ selectivity offered by the two-step process, for carrying out the process on a technical scale preference will often be given to a catalyst for the second step which, though not having the highest activity and C.sub.5.sup.+ selectivity, is the most stable.
Since the steam reformation of C.sub.4.sup.- hydrocarbons leads to the formation of a H.sub.2 /CO mixture having a H.sub.2 /CO molar ratio higher than 2, whilst Fischer-Tropsch catalysts have a H.sub.2 /CO consumption ratio of at most about 2, the excess hydrogen formed during the process will have to be removed in order to prevent H.sub.2 build-up in the system when the two-step process is carried out with use of recycle. Besides, when carrying out the two-step process using recycle, the part of the steam added during the steam reforming which has remained unconverted, as well as steam formed as by-product in the second step, should also be removed during the process. The quantity of hydrogen to be removed is dependent on the H/C atomic ratio of the feed for the first step, the CO-shift activity of the catalyst used in the second step and the degree of CO.sub.2 formation during the steam reforming. On the assumption of a stoichiometric conversion of the feed during the steam reforming according to the equation C.sub.n H.sub.2n+2 +nH.sub.2 O.fwdarw.nCO+(2 n+1)H.sub.2, the synthesis gas obtained will have a higher H.sub.2 /CO molar ratio according as the feed for the first step has a higher H/C atomic ratio, and therefore more hydrogen will have to be removed during the process. For instance, starting from methane (having n=1) as feed for the steam reforming, a synthesis gas can be obtained by the reaction given above which has a H.sub.2 /CO molar ratio of (2n+1)/n=3. According as the catalyst used in the second step has higher CO-shift activity, a larger part of the amount of CO present in the synthesis gas will react with the water formed as by-product in the hydrocarbon synthesis according to the equation CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2, leading to an increase of the H.sub.2 /CO molar ratio, and therefore more hydrogen will have to be removed in the process. As regards the formation of carbon dioxide during the steam reforming, the following may be observed. As described hereinbefore, when the steam reforming reaction proceeds stoichiometrically, there will be formed from each g atom C present in the feed, one g mol CO. However, in actual practice it is seen that depending on the conditions under which the steam reforming is carried out, part of the carbon present in the feed is converted into carbon dioxide. On account of this side reaction the synthesis gas obtained has a H.sub.2 /CO molar ratio which is higher than at a stoichiometric development of the steam reforming. In the process more hydrogen will therefore have to be removed according as more carbon dioxide is formed in the steam reforming.
In order to keep the amount of hydrogen to be removed as small as possible when carrying out the two-step process with use of recycle starting from a feed with a given H/C atomic ratio, preference is given in the first place to the use in the second step of a catalyst with the highest possible H.sub.2 /CO consumption ratio. It is also preferred to add carbon dioxide to the feed for the first step in order to suppress carbon dioxide formation during the steam reforming. With a view to optimum utilization of the carbon present in the feed for the formation of carbon monoxide, it is preferred to use carbon dioxide present in the reaction product of the first step for this purpose. This product can be scrubbed to separate the carbon dioxide, which can be recycled to the steam reforming. This procedure is attended with serious drawbacks. Apart from the fact that the removal of carbon dioxide from a gas stream by the use of scrubbing, in which often an amine-solution is used from which the carbon dioxide must be separated afterwards, is a rather costly process when carried out on a technical scale, it is a consequence of this mode of carbon dioxide removal that the carbon dioxide which originally was at the pressure level required in the process, is at an atmospheric pressure after separation and has to be re-pressurized to the pressure level of the process before it can be introduced into the steam reforming.
It would of course be much more attractive if it were possible to keep the carbon dioxide formed in the reaction product and not to separate it until after the second step. By dividing the reaction product of the second step into a liquid fraction substantially consisting of C.sub.5.sup.+ hydrocarbons and water and a gaseous fraction substantially consisting of unconverted hydrogen and carbon monoixde, C.sub.4.sup.- hydrocarbons and carbon dioxide, and recycling the gaseous fraction to the steam reforming, there could be created a carbon dioxide recycle without there being the need to depressurize and then again to pressurize the carbon dioxide. However, the use of such a process on a technical scale is to a great extent dependent on the influence which carbon dioxide has on the catalyst in the second step. As stated hereinbefore, in the two-step process with use of recycle stability is a parameter of particular importance; however, this does not imply that any negative influence which carbon dioxide may have on the activity or C.sub.5.sup.+ selectivity is regarded unimportant.
In order to get a fair knowledge of the influence of carbon dioxide on the performance of Fischer-Tropsch catalysts, an investigation was carried out in which these catalysts were used for the conversion of gas mixtures some containing carbon dioxide in addition to H.sub.2 and CO, some not. It was found that the presence of carbon dioxide in the H.sub.2 /CO mixture lessens the activity of these catalysts, the decrease becoming larger as the mixture contained more carbon dioxide. It is true that by increasing the severity of the reaction conditions--notably raising the temperature and/or pressure--in the presence of carbon dioxide an activity level could be attained which corresponded with that of a carbon dioxide-free operation, but this was attended with a loss of the catalysts' stability, which became larger as severer reaction conditions were used. It was further found that the C.sub.5.sup.+ selectivity of these catalysts was barely influenced by the presence of carbon dioxide in the H.sub.2 /CO mixture. As regards the stability the investigation yielded a surprising finding. In contrast with other Fischer-Tropsch catalysts whose stability--as well as C.sub.5.sup.+ selectivity--was barely influenced by the presence of carbon dioxide, there was a certain group of cobalt catalysts whose stability was found to be considerably increased by the presence of carbon dioxide, the increase being larger according as the mixture contained more carbon dioxide. The Fischer-Tropsch catalysts displaying this surprising behavior comprise silica, alumina or silica-alumina as carrier material and cobalt together with zirconium, titanium and/or chromium as catalytically active metals in such quantities that in the catalysts there are present 3-60 pbw cobalt and 0.1-100 pbw zirconium, titanium and/or chromium per 100 pbw carrier material. The catalysts are prepared by depositing the metals concerned by kneading and/or impregnation on the carrier material. For further information on the preparation of these catalysts by kneading and/or impregnation reference is made to U.S. patent application Ser. No. 594,618 filed on Mar. 29, 1984, which is incorporated herein by reference.
When a cobalt catalyst belonging to the afore-mentioned class is used for the conversion of a H.sub.2 /CO mixture containing no carbon dioxide, this catalyst is seen under the given reaction conditions to have not only high stability and C.sub.5.sup.+ selectivity, but also very high activity. When the same catalyst is used under similar reaction conditions for the conversion of a gas mixture which, in addition to H.sub.2 and CO, contains carbon dioxide, a decrease in activity is seen, as was remarked hereinbefore. The decrease is smaller, by the way, than the decrease observed for other Fischer-Tropsch catalysts when the same amount of carbon dioxide is added to the gas mixture to be converted. However, in addition to the decrease in activity the cobalt catalysts show a considerable increase in stability. In view of the very high activity level of the present cobalt catalysts some loss of activity in return for a considerable increase in stability is quite acceptable for an operation carried out on a technical scale. Another option is to raise the activity to its original level by increasing the severity of the reaction conditions; this is coupled with some loss of stability. However, it has surprisingly been found that this loss of stability is amply compensated for by the increase in stability due to the presence of carbon dioxide. This means that when the cobalt catalysts belonging to the above-mentioned class are used for converting a carbon dixoide containing H.sub.2 /CO mixture, a degree of activity can be realized which is very similar to that seen in the carbon dioxide free operation, whilst the stability is higher. These special properties combined with a very high H.sub.2 /CO consumption ratio of about 2 render the cobalt catalysts eminently suitable for use in the second step of said two-step process carried out with use of recycle.